Addressed following TODO (Thanks to Freek for the tip): 
//TODO: can we use Collections.sort with a custom comparator here? 
ReportUnit.java now implements Comparable<ReportUnit> interface.
Added a comparator in ReportUnit.java to compare report units.
Removed compareTo(thisUnit, otherUnit) method from ReportUnit.java. 
ViewerFrame.java now uses Collections.sort() method to sort reports.